Vertical probes are typically used to make electrical contact between the chip pads of chips carried on wafers and a testing system or machine used for device testing. The device chip pads are densely spaced, and the function of the vertical probe is to "fan-out" or enlarge this dense spacing to accommodate the larger spacing between adjacent electrical connectors found in most testing equipment.
A typical vertical probe is the cobra probe described in U.S. Pat. No. 4,027,935. This probe consists of upper and lower dies, a housing supporting the two dies, and multiple contacts formed from thin wire, e.g. 4 to 5 mils in diameter. The contacts are generally sharpened to a fine point, like a pencil point, for improving the ability of the probe tip to penetrate aluminum oxide on the chip pads. In addition, the contacts are mounted in the dies in a manner such that minor differences in the topography of different chip pads are accommodated by the inherent resiliency of the contacts.